People across England and Wales say their homes and buildings are being swarmed by ladybirds.
Ladybird experts suggest the hot summer has boosted numbers of the invasive Harlequin species, which first arrived in the UK in 2004.
As the temperature cools, the Harlequin hibernates for the winter in buildings - including homes.
Scientists say the ladybirds are mostly harmless and can be left alone if they are found in your house.
Swarms of posts have appeared on social media documenting the ladybird invasion, with little red bugs seen crawling over doors and windows in people's homes.
Professor Helen Roy at the Centre for Ecology and Hydrology, who organises the UK Ladybird Survey, said reports had started in the north of England where the weather first turned cool, before spreading down the country.
She said: "It's quite a wildlife spectacle to see."
Difficult to see but there are thousands of ladybirds out today. I can smell them in the air. pic.twitter.com/IfcnyfgEhq— Alex Murray (@AlexRJMurray) October 5, 2018
The hot summer was likely to have boosted the insect's numbers, although she said the survey data was not yet complete.
Native to Asia, the Harlequin ladybird was first seen in the UK 14 years ago, and is now the second most common ladybird species, seen across England and parts of Wales.
But its arrival has caused concern that it is displacing some native species, such as the two-spot ladybird.
Peter Brown, another ladybird survey organiser who is also a senior lecturer at Anglia Ruskin University, says that although the Harlequin species is usually bigger, it isn't always easy to distinguish from the native varieties in the wild.
But he said that ladybirds seen crawling into homes are likely to be this invader, as other species tend to hibernate in trees or leaf litter.
He said: "If you're getting large numbers of ladybirds coming into a building, they are very likely to be Harlequins."
Dr Brown said the Harlequin's annual swarming in autumn before hibernation had led to the nickname of the Halloween ladybug in the US.
Oh no ... the Harlequin Ladybird invasion has started again! pic.twitter.com/t3VJBxmOXy— Andy Scargill (@andyscargill) October 4, 2018
Dr Brown said Harlequin ladybirds sometimes bite people if no food is available, usually leaving a small bump and sting. In a few cases, people have had severe allergic reactions.
But he said that if you do spot Harlequins in your home, it might be easiest to leave them alone - when disturbed they secrete a yellowish substance that can stain furnishings.